1. Field of the Invention
The present invention relates to a heat resistant alloy member which is used at high temperatures or in corrosive environments at high temperatures.
2. Description of the Prior Art
Techniques for improving the heat resisting property of structural members of gas turbines operated by high temperature gas have been investigated with a view for improving the thermal efficiency of gas turbine power stations. The establishment of the above-mentioned techniques is necessary for coal gasification power stations having higher cost of fuel in order to enhance its economically competitive ability against the other kind of power stations. There is therefore a demand for improved heat resisting alloy members in order to cope with the intention to increase the gas temperatures for gas turbine power stations. A principal method of providing members with heat resistance at higher temperatures is to develop new materials for forming such members. Among various types of metal materials, Ni or Co-based alloys have heat resistant temperatures of about 850.degree. C. On the other hand, ceramics have a sufficient heat resisting property for high temperatures but involve certain problems with respect to their toughness and so on, particularly when they are used in moving blades which serves as high-speed rotors. Thus another method of achieving the technique for improving a heat resisting property is to prevent any increase taking place in the temperature of the relevant members. An example of this method is the combination of cooling members and coating members with ceramics having a low degree of thermal conductivity. Such a coating is called a thermal barrier coating (referred to as "TBC" hereinafter). TBC comprises a base metal composed of a heat resisting alloy and a coating of ceramics having physical properties which are different in numerical value from those of the base metal. An important technical problem of TBC is thus to reduce the thermal strain and thermal stress produced owing to the difference in the numerical values of the physical properties between the base metal and the ceramic coating. In particular, damage such as separation or the like may occur in the ceramic coating layer owing to the thermal stress based on a cyclic heating from starting to stopping of a gas turbine. A known method of reducing thermal stress is the method in which an intermediary layer is provided which serves to reduce the difference in thermal expansion coefficient between the ceramic coating layer and the base metal composed of a heat resisting alloy. Such an intermediary layer is disclosed in, for example, Japanese Patent Laid-Open No. 211362/1987. The intermediary layer is generally a mixture layer comprising ceramics and a metal. Although the thermal expansion coefficient of such a mixture layer depends upon the mixing ratio used, it is generally considered that the mixture layer should have a thermal expansion coefficient of a value midway between those of the ceramics and the metal. When this sort of mixture layer is interposed between a ceramic layer and a base metal, a function of reducing thermal stress can, as a matter of course, be expected.
On the other hand, since the ceramic coating layer used in TBC is mainly formed by spray coating, it is a porous substance. This porous ceramic coating layer is capable of reducing thermal stress by itself by virtue of its porous structure. However, since the ceramic coating layer may be used in corrosive environments at high temperatures, high temperature oxidation or high temperature corrosion takes place in the mixture layer provided below the ceramic coating layer through the ceramic coating layer which consists of a porous substance. The inventors have thus conducted oxidation tests on mixture coating layers comprising ceramics and metals. Each of the test pieces employed was made by forming a mixture coating layer on a surface of a base metal and then removing the base metal to form a sample comprising a mixture. Each of the thus-formed test pieces was then subjected to an oxidation test under heating at 1000.degree. C. for 1000 hours in the atmosphere. As a result, internal oxidation proceeded to a significant extent in each of the test pieces comprising mixtures in the oxidation tests. It is thought that such internal oxidation proceeds through cavities present at grain boundaries in the coating layers which comprises the mixture of ceramic powder and metal powder and which is simply formed by laminating the two types of powder by spray coating. Such internal oxidation in a mixture layer proceeded to a significant extent in a ceramic-coated test piece comprising a ceramic coating layer, a mixture layer and a base metal. The ceramic layer in this ceramic-coated test piece became separated after a high-temperature oxidation test at 1000.degree. C. for 1000 hours. Thus the mixture layer provided for the purpose of reducing thermal stress cannot achieve the intended purpose. It is thought that the separation of the ceramic layer is caused by the thermal stress newly produced in the mixture layer owing to the internal oxidation in the mixture layer itself and by a reduction in the adhesion between the ceramic coating layer and the mixture layer owing to the oxidation at the boundary therebetween. Such a problem causes a reduction in the reliability of TBC. On the other hand, a thermal barrier effect required for TBC is increasingly improved as the working temperature of a gas turbine is raised. In other words, it is necessary to increase the thickness of the ceramic coating layer for the purpose of improving the thermal barrier effect. In this case, the thermal stress produced by a repeated heat load or the like is of course increased. It is therefore necessary to improve the durability of the ceramic coating by reducing the thermal stress produced in the ceramic coating layer owing to a repeated heat load or the like.
As described above, although TBC provided with a mixture layer is provided for the purpose of reducing the thermal stress produced between a ceramic coating layer and a base metal, the mixture layer does not always possess a sufficient function of reducing thermal stress under high temperature conditions because the resistance to oxidation of the mixture layer at high temperatures is inadequate. In addition, the mixture layer does not always possess sufficient corrosive resistance at high temperatures.